Add rust implementation of forward euler (#182)

* Add rust implementation of forward euler

* remove n's from the arguments, as they are obviously unnecessary

* rustfmt

Author: hsjoihs

CONTRIBUTORS.md

@@ -13,5 +13,6 @@ Chinmaya Mahesh
 Unlambder
 Kjetil Johannessen
 CDsigma
+hsjoihs
 DominikRafacz
 lulucca12

chapters/differential_equations/euler/code/rust/euler.rs

@@ -0,0 +1,31 @@
+fn main() {
+    let mut result = [0.0; 100];
+    let threshold = 0.01;
+    let timestep = 0.01;
+
+    solve_euler(timestep, &mut result);
+    println!("{}", check_result(&result, threshold, timestep));
+}
+
+fn solve_euler(timestep: f64, result: &mut [f64]) {
+    let n = result.len();
+    if n != 0 {
+        result[0] = 1.0;
+        for i in 1..n {
+            result[i] = result[i - 1] - 3.0 * result[i - 1] * timestep;
+        }
+    }
+}
+
+fn check_result(result: &[f64], threshold: f64, timestep: f64) -> bool {
+    let mut is_approx: bool = true;
+    for (i, val) in result.iter().enumerate() {
+        let solution = (-3.0 * i as f64 * timestep).exp();
+        if (val - solution).abs() > threshold {
+            println!("{}    {}", val, solution);
+            is_approx = false;
+        }
+    }
+
+    return is_approx;
+}

chapters/differential_equations/euler/euler.md

@@ -107,6 +107,9 @@ So, this time, let's remove ourselves from any physics and instead solve the fol
 {% sample lang="cpp" %}
 ### C++
 [import, lang:"c_cpp"](code/c++/euler.cpp)
+{% sample lang="rs" %}
+### Rust
+[import, lang:"rust"](code/rust/euler.rs)
 {% sample lang="elm" %}
 ### Elm
 [import:44-54, lang:"elm"](code/elm/euler.elm)